The present invention relates to a rotation detecting device for detecting a rotation of a detection subject using a magnetic force.
Patent Reference 1 has disclosed a conventional rotation detecting device. FIG. 13 is a schematic perspective view showing the conventional rotation detecting device 101 disclosed in Patent Reference.
Patent Reference: Japanese Patent Publication No. 2001-194182
As shown in FIG. 13, the conventional rotation detecting device 101 includes a plurality of magnets 102 and a detection element 103. Each of the magnets 102 is fixed on an outer circumference portion of a rotational member 104 having a cylindrical shape. Further, each of the magnets 102 is formed of a permanent magnet, and is magnetized in a direction in parallel to a rotational axis 105 of the rotational member 104. Further, the magnets 102 are arranged along an outer circumference direction of the rotational member 104 such that magnetized directions thereof become opposite alternately.
In the conventional rotation detecting device 101, the detection element 103 is disposed near the outer circumference portion of a rotational member 104. Further, the detection element 103 is formed of a magnetic element 106 having a wire shape and capable of generating a large Barkhausen effect. A coil 107 is wound around the magnetic element 106.
In the conventional rotation detecting device 101, the magnetic element 106 extends in a direction in parallel to the rotational axis 105 of the rotational member 104. Further, the magnetic element 106 is magnetized in a magnetization direction in parallel to the rotational axis 105 of the rotational member 104, and the magnetization direction is reversed when an external magnetic field is applied.
In the conventional rotation detecting device 101 disclosed in Patent Reference, the rotational member 104 is rotated together with the magnates 102, and the detection element 103 is stationary at one location. Accordingly, when the rational member 104 is rotated, the magnets 102 sequentially pass through near the detection element 103, so that an alternate magnetic field is applied to the wire-shaped magnetic element 16. During this process, every time when one of the magnets 102 gets close to the detection element 103, the magnetization direction of the detection element 103 is reversed due to the magnetic field generated by the one of the magnets 102. As a result, the coil 107 generates a pulse signal.
In the conventional rotation detecting device 101 disclosed in Patent Reference, when a radius of the rotational member 104 is decreased so that a size of the conventional rotation detecting device 101 is reduced, an outer circumference of the rotational member 104 is decreased. As a result, an interval of the magnets 102 disposed along the outer circumference portion of the rotational member 104 is also decreased.
Further, in the conventional rotation detecting device 101, when the number of the magnets 102 is increased, the number of the pulses per rotation is increased. Accordingly, it is possible to improve detection accuracy of the conventional rotation detecting device 101. However, when the number of the magnets 102 is increased, the interval of the magnets 102 disposed along the outer circumference portion of the rotational member 104 is decreased.
Further, in the conventional rotation detecting device 101, when one of the magnets 12 approaches the detection element 103, a magnetic flux of the one of the magnets 12 is considered to pass through paths P51, P52, and P53 as shown in FIG. 13. The path P51 passes from an N pole of the one of the magnets 12 to an S pole of the one of the magnets 12 through the magnetic element 106 in a direction in parallel to the rotational axis 105. The path P52 passes from the N pole of the one of the magnets 12 to an S pole of another one of the magnets 12 situated adjacent to the one of the magnets 12. The path P53 passes from an N pole of another one of the magnets 12 situated adjacent to the one of the magnets 12 to the S pole of the one of the magnets 12.
As described above, in the conventional rotation detecting device 101, when the interval of the magnets 102 disposed along the outer circumference portion of the rotational member 104 is decreased, the N poles and the S poles of the two adjacent ones of the magnets 102 get close to each other. Accordingly, the magnetic flux tends to concentrate in the paths P52 and P53 that do not pass through the magnetic element 106, and the magnetic flux tends not to concentrate in the path P51 that passes through the magnetic element 106.
As a result, even when the magnets 102 sufficiently get close to the detection element 103, the magnetization direction of the magnet element 106 may not be reversed, thereby causing malfunction in which the coil 107 fails to generate the pulse. If the coil 107 does not properly generate the pulse, it is difficult for the conventional rotation detecting device 101 to detect accurately the rotation of the rotational member 104. In other words, it is difficult for the conventional rotation detecting device 101 to improve the rotation detection accuracy, as well as to reduce the size thereof.
In view of the problems of the conventional rotation detecting device described above, an object of the invention is to provide a rotation detecting device capable of solving the problems. According to the rotation detecting device of the present invention, it is possible to improve the detection accuracy of the rotation detecting device, as well as to reduce the size thereof.
Further objects and advantages of the invention will be apparent from the following description of the invention.